This invention relates to filter systems, and more particularly to improved means for controlling the backwash in filtration systems of the type described more nearly to maintain constant as nearly as possible the pressure of the fluid flowing in the system.
Regardless of the exact construction of a given filter element, the fact remains that the debris which it removes from the fluid that is being filtered progressively builds up on, or in, the pores or filter openings in the element, thereby causing more and more of these openings to become closed off. As a consequence the resistance to the flow of fluid through the filter increases, and as a corollary the volume of the fluid flowing through the filter usually decreases. This creates a problem, because as a general rule it is most desirable to maintain a constant flow or volume of properly filtered fluid through a system.
A variety of solutions have been proposed to obviate this problem. Among the most fundamental is the use of self-cleaning filters, which are intermittently backwashed to remove accumulated solids or debris which impede the flow of fluids through the filter. This process, of course, requires periodic removal of the respective filter elements from the system in order to permit them to be backwashed or cleaned, before being returned to operation in the system. (See, for example, U.S. Pat. Nos. 2,366,903 and 2,828,862.) As a supplement to this method, it is customary also to employ a plurality of different filters, certain of which are being backwashed while others are being utilized in the system, thus minimizing the shut-down time of the system.
As disclosed for example in U.S. Pat. No. 3,172,846, it has been customary with prior such backwash operations automatically to initiate backwashing whenever the resistance to fluid flow through the filter reaches a certain, predetermined setpoint. Typically this is done by monitoring the pressure drop across the filter--i.e., the pressure differential between the inlet and outlet of the filter--and initiating backwash when this differential reaches a predetermined set value. After being cleaned the filter is reintroduced into the system where it remains in operation until the set or predetermined pressure differential across the filter has once again been reached, at which time the backwashing operation once again commences automatically.
One of the major disadvantages of backwash operations of the type described is that, as a matter of fact, each backwash or backflush cycle does not completely clean the associated filter element, so that over a prolonged period of use the filter element eventually becomes completely clogged or useless.
For example, assuming that a fluid, such as a coolant, is pumped through a multiple tube pressure filter at a gauge pressure of 50 p.s.i. when the tubes are new and perfectly clean; and assuming that the tubes are set to be backwashed automatically when the pressure differential across the filter reaches 10 p.s.i. (i.e., 50 p.s.i. on the outsides of the tubes and 40 p.s.i on the inside), then when the tubes are backflushed to atmosphere, the pressure at the insides of the tubes (40 p.s.i.) will be four times greater than the 10 p.s.i. differential which triggered the backwash. Each time that the backwash occurs some additional debris remains on the filter tubes, so that when they are next reinserted into the system for filtering purposes the initial or starting differential pressure will be higher than at the start of the preceding filtering sequence. In other words, after filtering and backwashing the tubes cannot be returned to their original state of cleanliness. Obviously, therefore, each time the tubes are backwashed and reintroduced into the system, the system will start out with a higher, residual differential pressure across its filter tubes, and as a consequence its backwash operation will be triggered that much sooner. In any case, whenever the backwash occurs the ratio of the backflush pressure (40 p.s.i.) to the preset differential pressure (10 p.s.i.) will be at the constant ratio of 4 to 1.
Still another disadvantage of such prior backwash systems is that, for the most part, as the pressure differential slowly builds up to the preset triggering value (for instance 10 p.s.i.)between each backwash cycle, there is a corresponding drop in the downstream pressure and volume of the fluid flowing through the system. As a consequence there is an intermittent and undesirable rising and falling in the overall volume and pressure of the fluid flowing in the system.
While it has been suggested that the interval of time between backwashes diminishes successively with each additional backwashing operation, this presupposes that the fluid being filtered remains relatively consistent as far as the nature and content of the impurities that are being removed from the fluid by the filter. In practice, however, the consistency of the fluid being filtered may well vary rather widely, in which case the intervals of time between successive backwashes may differ considerably, depending for example upon the quantity of impurities or debris contained in the fluid during any given filtering sequence.
It is an object of this invention, therefore, to provide an improved method for controlling the backwash of filters thereby better to stabilize the pressure of fluid flowing in filtration systems of the type described herein.
Still another object of this invention is to provide for systems of the type described improved control means of which operates automatically to vary the set point or differential pressure at which a backwash operation occurs, depending upon the rate at which the associated filter becomes clogged.
A further object of this invention is to provide for a system of the type described, a method of comparing the rate of clogging of the associated filter unit with an established base rate or curve, which is representative of the filter in its new or perfectly clean state, and automatically adjusting the value of the preset backwash pressure either up or down in proportion to the difference between the actual rate of clogging and the indicated base rate.
A more specific object of this invention is to provide a method of the type described which effects reduction in the preset value of the backwash in amounts which are inversely proportional to the rate of change in the differential filter pressure indicated by the base curve.